Electrochimica Acta 109 (2013) 720–731
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis of Bi2 WO6 nanoparticles and its electrochemical properties
in different electrolytes for pseudocapacitor electrodes
V.D. Nithya a , R. Kalai Selvan a,∗ , D. Kalpana b,∗ , Leonid Vasylechko c , C. Sanjeeviraja d
a
Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India
Electrochemical Power Systems Division, Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India
Semiconductor Electronics Department, Lviv Polytechnic National University, 12 Bandera Street, Lviv 79013, Ukraine
d
Department of Physics, Alagappa Chettiar College of Engineering and Technology, Karaikudi 630 004, Tamil Nadu, India
b
c
a r t i c l e
i n f o
Article history:
Received 18 April 2013
Received in revised form 26 June 2013
Accepted 16 July 2013
Available online xxx
Keywords:
Bismuth tungstate
Rietveld analysis
Cole–Cole plot
Charge–discharge analysis
Pseudocapacitors
a b s t r a c t
Nanosized Bi2 WO6 particles were successfully synthesized by sonochemical method with an objective to
develop an inexpensive and eco-friendly electrode material for supercapacitors. The prepared material
was subjected to various thermal, structural, morphological, compositional, electrical and electrochemical studies. Bi2 WO6 nanoparticle with homogeneous distribution was achieved through sonochemical
process. The lattice parameter and atomic positions of Bi2 WO6 structure were refined through Reitveld
analysis. The electrochemical performance of Bi2 WO6 nanoparticles was investigated in various aqueous
electrolytes such as 1 M NaOH, 1 M LiOH, 1 M Na2 SO4 , 1 M KOH and 6 M KOH solutions. Among these,
the material exhibited an enhanced electrochemical performance in KOH electrolyte due to its smaller
hydration sphere radius, high ionic mobility and lower equivalent series resistance. The charge–discharge
studies rendered a specific capacitance of 608 F/g in 1 M KOH at a current density of 0.5 mA/cm2 . Bi2 WO6
exhibited an excellent coulombic efficiency and specific capacitance of around 304 F/g at 3 mA/cm2 in
the potential range from −0.9 to 0.1 V vs Hg/HgO in 1 M KOH electrolyte. The above results assured that
Bi2 WO6 could be utilized as suitable negative electrode material for supercapacitor applications and 1 M
KOH could be its desirable electrolyte.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Ultracapacitors or supercapacitors have gained increasing interest in recent times due to its high power density (1–10 kW/kg)
than the conventional battery systems (150 W/kg). The charges
are stored at the electrode/electrolyte interface and outer surface
than the bulk of the electrode which favours rapid charge/discharge
rates and this being the main reason for having high power density of supercapacitors [1]. Moreover the life time is expected to be
around lakhs of cycles and further the supercapacitors possess long
shelf life, high efficiency, eco-friendly and safe to use. The major
application of this supercapacitor includes hybrid electric vehicles where they are used in conjunction with the batteries. Other
applications include mobile phones, cameras, UPS (Uninterruptible
Power Supplies), braking energy systems, etc. [2]. The major drawback of these capacitors is its low energy density. The possible ways
that could amend the energy density (given by 1/2 CV2 ) is (i) by
either enhancing the specific capacitance or (ii) increasing the cell
∗ Corresponding authors.
E-mail addresses: [email protected] (R. Kalai Selvan),
[email protected] (D. Kalpana).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.07.138
voltage of the material. The amount of energy that are stored in a
unit mass/volume (energy density) is determined by the specific
capacitance of the electrode material, electronic/ionic conductivity
of the electrode and as well as the ionic conductivity and stability
window of the electrolyte.
Based on the charge storage mechanism, supercapacitors are
classified into two types such as electric double layer capacitors
(EDLC) and pseudocapacitors. In EDLC’s, the charges are stored
through the adsorption/desorption process [3]. The carbon based
materials such as activated carbon, carbon aero gels, and carbon
nano tubes are employed as the EDLC electrodes. On the other hand,
in pseudocapacitors the charges are stored through the Faradic
reactions. The transition metal oxides and conducting polymers are
widely used as electrodes. These pseudocapacitors possess higher
energy density due to the enhanced capacitance compared with
EDLC’s [4]. Hence, a numerous reports have been explored using
transition metal oxides, hydroxides and nitrides that includes RuO2
[5], CuO [6], TiO2 [7], Co3 O4 [8], MnO2 [9], SnO2 [10], NiO [11], ␣NiMoO4 [12], NiCo2 O4 [13], Co3 S4 [14], V2 O5 [15], VN [16], Ni(OH)2
[17], ␣-Fe2 O3 [18], ␣-Co(OH)2 [19], etc., as the active electrodes for
pseudocapacitors.
The work will be of great importance since efforts was made to
employ Bi2 WO6 as an outstanding functional material. It was well
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
documented that, Bi2 WO6 possess piezoelectric, pyroelectric, ferroelectric, non-linear dielectric susceptibility, catalytic and oxide
anion conducting behaviour [20]. They are used as an excellent
visible light photo catalyst for the degradation of pollutants and
organic dyes [21–32], electrode material for Li-ion batteries [33]
and an oxygen evolution half reaction for water splitting, etc. [34].
Bi2 WO6 belongs to Aurvillius layered structure with a general formula of Bi2 An−1 Bn O3n+3 (where n = 1) in which A site comprises of
Ca, Sr, Na, Pb, K, etc., and B site with Ti, Nb, Mo, W, Fe ions, respectively. It crystallizes in orthorhombic structure with corner sharing
WO6 octahedral layer with Bi2 O2 2+ layers sandwiched between
WO6 octahedral layers [35].
Nowadays a lot of efforts including synthesis methods and conditions have been adopted to tune the morphology of the particles
based on the specific applications since it is strongly believed
that the close association between the morphology and properties [24,36]. In this regard, Bi2 WO6 nanostructures was synthesized
by different methods such as hydrothermal [36], ultrasonic spray
pyrolysis [37], refluxing with ethylene glycol [38], electro spinning
[39], sol–gel [21], spin coating [40], solid state reaction [35], pulsed
laser deposition [41], ultrasound assisted synthesis [42], etc. Each
method has its own advantages and disadvantages. Here, we have
used sonochemical method for the synthesis of Bi2 WO6 since it
offers the advantages like high crystalline products formed with
homogeneous size distribution, and comparable less reaction time,
etc. It is very simple, efficient, economical and environmentally
benign method. In addition to that, the method initiates new reactions that are difficult to carry out in normal conditions and also
avoid calcinations at higher temperatures. Due to the application
of ultrasound, chemical changes occurs in the liquid medium due to
acoustic cavitation which involves formation, growth and collapse
of bubbles, producing hot spots having temperature of the order of
5000 ◦ C and 1800 atmospheric pressure [43,44].
In the present work, Bi2 WO6 material was synthesized by
sonochemical method and characterized using various techniques
such as TG/DTA (Thermo gravimetric/differential thermal analysis), XRD (X-Ray Diffraction) with Rietveld analysis, FESEM
(Field-Emission Scanning Electron Microscope), A.c. impedance
spectroscopic technique, respectively. The electrochemical performance was investigated using cyclic voltammetry (CV),
Galvanostatic charge–discharge and electrochemical impedance
spectroscopy (EIS) for its application as pseudocapacitor. In general, pseudocapacitive performance of Bi2 WO6 depends mainly on
the compatibility of the electrode with the electrolyte. Upon the
usage of electrolytes, there should not be any electrode degradation
or corrosion of current collectors. Here we have focused mainly in
examining the effect of electrolytes on pseudocapacitive behaviour
of Bi2 WO6 . The electrochemical performance is studied in different
aqueous electrolytes.
2. Experimental methods and materials
All the chemicals were of analytical grade and used without any further purification. For 1 g Bi2 WO6 , the stoichiometric
amounts, i.e. 0.4726 g of Bi (NO3 )3 ·5H2 O and 1.3903 g of Na2 WO4
were taken. The Bi(NO3 )3 ·5H2 O was dissolved in dil.HNO3 and
Na2 WO4 in 10 ml of double distilled water individually. The
Bi(NO3 )3 ·5H2 O solution was magnetically stirred and the sodium
tungstate solution was added drop-wise gradually (2 ml/min)
into the above solution. Initially the pH of the solution was
found to be 1 and was varied to pH 7 by adding sufficient
amount of aqueous ammonia. The white solution was stirred for
about 15 min and placed inside the ultrasonication set up containing Ti horn (1/2 inch diameter). The solution was irradiated
with high intensity ultrasound (20 kHz, 30 W/cm2 ) for about two
721
hours to get the colloidal white precipitate. A constant temperature of 40 ◦ C was maintained throughout the experiment by
placing the water bath in the sonication cell. The obtained precipitate was washed several times with double distilled water
and ethanol to remove impurities and dried overnight in air at
100 ◦ C.
The TG/DTA analysis (Thermo gravimetric/differential thermal
analysis) was performed using Perkin Elmer STA 6000 thermobalance at a heating rate of 15 ◦ C/min in a static air atmosphere in
the temperature range between 40 and 800 ◦ C. The X-Ray Diffraction (XRD) analysis was carried out using BRUKER D8 Advance with
CuK␣ radiation. In order to perform the detailed structural analysis,
Rietveld refinement was carried out. The Rietveld refinement was
performed using WinCSD programme package. The morphology of
the material was analyzed by (Field Emission Scanning Electron
Microscope) FESEM using Leo Supra 55, Genesis 2000, Carl Zeiss
equipped EDX (Energy Dispersive X-ray Spectroscopy). The electrical conductivity measurement was carried out using computer
controlled impedance analyzer HIOKI 3532 LCR HITESTER in the
frequency ranging from 50 Hz to 10 kHz. For conductivity measurements, the powder was pressed in to pellet of 1 cm diameter at a
pressure of 150 kg/cm2 and sandwiched between two electrodes of
sample holder.
The working electrode for studying the electrochemical performance was prepared by mixing the active material, carbon black,
PVDF (polyvinylidene fluoride) in the weight ratio of 80:10:10 using
NMP (N-methyl 2 pyrrolidione) as solvent to form slurry. Then
10 ␮l of slurry was coated onto a stainless steel electrode of area
1 cm2 . Finally, the loaded active mass was calculated to be 1 mg
without carbon black and PVDF. The coated slurry was air dried
at 50 ◦ C for overnight. The cyclic voltammetry, charge–discharge,
electrochemical impedance analysis was carried out using SP-150
BIO-LOGIC science workstation. Three electrode configuration was
used for electrochemical characterization utilizing platinum as
counter electrode, Hg/HgO & SCE was used as the reference electrode. Different electrolytes such as LiOH, NaOH, KOH and Na2 SO4
was taken to study the effect of various electrolytes, role of cations
and anions contributing to the electrochemical performance and
finally the effect of electrolyte concentrations. The stability of the
material was assured using the galvanostatic charge–discharge
curves for 500 cycles. The electrochemical impedance spectrum was studied in the frequency range between 10 mHz and
1 MHz.
3. Results and discussion
3.1. Thermal analysis
The thermal stability of the as-prepared Bi2 WO6 is determined
using TG/DTA analysis by applying temperature from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min. Fig. 1 shows
the TGA/DTA curves of Bi2 WO6 . The TG curve shows two different weight losses with increase in temperature. The observed 12%
weight loss in the temperature range between 50 and 160 ◦ C is
due to the loss of hydroxyl groups and 62% of loss between 260
and 580 ◦ C is due to the removal of volatile organic impurities
[45]. It is to be noted that above 580 ◦ C, there is no significant
weight loss observed which indicates that all the organic compounds are decomposed before this temperature. The DTA curve
shows an endothermic peak at 125 ◦ C which attributes to the loss
of water contents present in the material. The other endothermic
peak at 350 ◦ C is due to the decomposition of the volatile organic
compounds. The exothermic peak at around 500 ◦ C results from
formation of Bi2 WO6 material and this behaviour are in concordant
with the results of Zhang et al. [46].
722
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
-400
80
Endo -300
60
-200
40
-100
Table 1
Crystallographic data for Bi2 WO6 (space group Pca21 , Z = 4; a = 5.4379(2) Å,
b = 16.4262(6) Å, c = 5.4549(2) Å, V = 487.30(5) Å3 , = 9.511(1) g/cm3 ).
Heat Flow / mW
Weight Loss (%)
100
20
Atoms, sites
x
y
z
Biso/eq , Å2
Bi(1), 4a
Bi(2), 4a
W, 4a
O(1), 4a
O(2), 4a
O(3), 4a
O(4), 4a
O(5), 4a
O(6), 4a
0.5219(4)
0.4847(3)
0.0118(5)
0.062(3)
0.253(4)
0.238(4)
0.710(4)
0.207(3)
0.567(4)
0.4213(2)
0.0766(2)
0.2465(2)
0.1318(13)
0.997(2)
0.500(2)
0.2343(12)
0.262(2)
0.3539(13)
0.9703(5)
0.9798(5)
0.0000*
0.072(4)
0.266(4)
0.260(4)
0.251(4)
0.337(3)
0.567(4)
1.13(8)
0.46(7)
0.45(8)
1.2(6)
0.9(5)
1.1(5)
0.3(4)
1.1(5)
0.4(5)
0
Anisotropic displacement parameters
B22
B33
B12
B11
0
100
200
300
400
500
600
700
800
Bi(1), 4a
1.13(11)
1.16(7)
0.50(11)
0.47(5)
Bi(2), 4a
0.63(12)
0.45(5)
W, 4a
RI = 7.49, RP = 9.90, RWP = 11.42%
o
Temperature / C
Fig. 1. TG/DTA curves of as-prepared Bi2 WO6 .
*
1.1(2)
0.4(2)
0.3(2)
−0.06(13)
−0.03(9)
0.03(8)
B13
B23
−0.0(2)
0.2(2)
0.0(3)
0.2(3)
0.0(2)
−0.1(3)
z – coordinate of W atoms were fixed at 0.
3.2. Structural analysis
(1 1 3)
The XRD patterns of as prepared and calcined Bi2 WO6 synthesized by the facile sonochemical method are shown in Fig. 2. Fig. 2a
shows the XRD pattern of as-prepared Bi2 WO6 . It is observed that
the XRD pattern of the material seems to be amorphous without
any sharp peaks but it signifies the formation of Bi2 WO6 structure.
The XRD pattern of 350 ◦ C calcined Bi2 WO6 is shown in Fig. 2b.
As the temperature is raised to 350 ◦ C, still the crystalline peaks
are not observed. But the crystallinity of the material is slightly
increased when compared with the as-prepared samples due to
the removal of water. When the material is calcined at 500 ◦ C, the
crystallinity of the sample is well established which is proven by
the sharp peaks as observed from Fig. 2c. At this temperature all the
organic compounds would have been removed and the results are
in consistence with the TG/DTA results. The XRD pattern demonstrates sharp and well defined peaks corresponding to the Bi2 WO6
phase with orthorhombic russellite structure. The lattice parameter
values deduced from the XRD peaks using the CELREF software are
in concordant with the standard JCPDS data file no 73-2020. However a detailed examination of X-ray powder diffraction pattern
shows that besides the main Bi2 WO6 phase, the material contains
traces of ı-modification of bismuth oxide Bi2 O3 (JCPDS Data file no
16-654, 27-52, 43-447).
In order to characterize the obtained crystalline material more
precisely the full profile Rietveld refinement was performed using
(1 3 9)
(4 2 0)
(0 0 12)
(1 1 9)
(2 2 6)
Intensity (a.u.)
(0 0 6)
(0 2 6)
(c)
(b)
(a)
10
20
30
40
50
60
70
2 (Degree)
Fig. 2. XRD pattern of Bi2 WO6 calcined at (a) 100 ◦ C (b) 350 ◦ C (c) 500 ◦ C.
80
WinCSD programme package [47]. It is necessary to note that there
was a permanent discussion in the literature concerning the true
symmetry and space group of Bi2 WO6 structure. However, comprehensive analysis of the results of high resolution time of flight
neutron powder diffraction performed by Knight in 1992 clearly
shown that Bi2 WO6 adopts orthorhombic Pca21 structure with
the lattice parameters a = 5.4373, b = 16.4302 and c = 5.4584 Å [48].
These structural parameters of Bi2 WO6 reported in [48], as well as
the atomic positions in cubic ı-Bi2 O3 [49] were used as starting
models for the two-phase full profile Rietveld refinement.
Calculation of X-ray powder diffraction profiles based on these
two structural models revealed a good agreement with the experimental pattern in a whole 2 range of 10–140◦ . Simultaneous
refinement of unit cell dimensions of both phases together with
background and peak profile parameters and correction of absorption and instrumental sample shift shows an excellent fit between
calculated and experimental profiles. Further improvement of the
fit was achieved after refinement of the positional and anisotropic
displacement parameters of Bi and W atoms in the Bi2 WO6 structure. The atomic positions and displacement parameters in the
minor ı-Bi2 O3 phase were fixed at all stages of the refinement.
On the final step of refinement procedure, the positions and displacement parameters of oxygen atoms in Bi2 WO6 structure were
refined in a “soft” mode, which however did not improve the
fit and residuals. A “soft” mode refinement of the positions of
oxygen atoms in Bi2 WO6 structure is strongly required; otherwise the incorrect values of some atomic displacement parameters
and W O interatomic distances will be obtained. Similar chemically unreasonable oxygen–tungsten distance of 1.509 Å in Bi2 WO6
structure was obtained earlier in [48] from X-ray synchrotron powder diffraction data. The reason is the well-known fact that X-ray
diffraction technique does not allow to locate precisely the oxygen
atoms in the presence of the heavy strongly scattered species like as
W and Bi. Simultaneous full-profile two-phase Rietveld refinement
allowed to establish a relative amount of Bi2 WO6 (96.64 wt.%) and
ı-Bi2 O3 (3.36 wt.%) phases in the sample analyzed. Refined value
of the lattice parameter of the parasitic Bi2 O3 phase a = 5.568 Å is
higher than those of “pure” ı-Bi2 O3 (a = 5.525 Å, JCPDS file no 27-52)
and rather corresponds to the tungsten-containing Bi3.84 W0.16 O6.24
phase (a = 5.5632 Å, JCPDS file no 43-447).
Final values of the refined lattice parameters, positional and displacement parameters of atoms in Bi2 WO6 structure, as well as
corresponding residuals are given in Table 1. Graphical results of the
Rietveld refinement are presented in Fig. 3. The refined structural
parameters of Bi2 WO6 presented in Table 1 are in good agreement
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
723
Fig. 3. (a) Graphical results of two-phase Rietveld refinement showing coexistence of Bi2 WO6 (96.64 wt.%) and Bi2 O3 -based phase (3.36 wt.%) in the sample analyzed.
Experimental XRD pattern is shown in comparison with the calculated patterns. The difference between measured and calculated profiles is shown as a curve below the
diagrams. Short vertical bars indicate the positions of diffraction maxima of Bi2 WO6 and Bi2 O3 -based phases (upper and lower rows, respectively). Insert shows cut-out of
the patterns.
with the data derived from neutron powder diffraction by Knight
[48] and Saiful Islam et al. [50]. Selected values of bond length and
bond angles in the Bi2 WO6 structure calculated from the refined
structural parameters are tabulated in Table 2.
Bi2 WO6 structure belongs to Aurivillus phases with general formula [Bi2 O2 ]2+ [An−1 Bn O3n+1 ]2− , consisting of intergrowth between
Bi2 O2 2+ sheets and [An−1 Bn O3n+1 ]2− perovskite-like slabs containing n = 1–8 layers [51]. In the case of Bi2 WO6 (i.e. n = 1) the structure
consists of alternating layers of corner-shared WO6 octahedra and
Bi2 O2 2+ (Bi2 O3 ) slabs lying perpendicular to the long [0 1 0] axis
Table 2
Selected bond lengths and bond angles with estimated standard deviations in parenthesis in the Bi2 WO6 structure.
Atoms
Distances (Å)
Atoms
WO6 polyhedra
W O(5)
W O(4)
W O(6)
W O(1)
W O(5)
1.79(2)
1.83(2)
1.85(2)
1.94(2)
2.14(2)
O(1)
O(1)
O(1)
O(4)
O(4)
W O(4)
(W O)ave
2.15(2)
1.95
O(4) W O(6)
O(5) W O(5)
O(5) W O(6)
W
W
W
W
W
Angles (degrees)
O(4)
O(5)
O(6)
O(4)
O(5)
Bi(1)O6 polyhedra
2.17(3)
Bi(1) O(3)
O(3) Bi(1) O(3)
Bi(1) O(3)
2.24(3)
O(3) Bi(1) O(6)
2.35(2)
2.48(2)
2.55(2)
2.56(2)
2.39
O(6) Bi(1) O(6)
Bi(1)
Bi(1)
Bi(1)
Bi(1)
(Bi(1)
O(3)
O(6)
O(6)
O(3)
O)ave
O(1) Bi(2) O(1)
O(1) Bi(2) O(2)
Bi(2) O(2)
O(2) Bi(2) O(2)
Bi(2)
Bi(2)
Bi(2)
(Bi(2)
O(1)
O(2)
O(1)
O)ave
2.42(2)
2.45(2)
2.52(2)
2.36
69.1(8), 67.1(8),
76.2(9), 74.3(9)
74.5(8), 80.7(8),
82.1(8), 70.3(7)
84.3(6)
3.3. Morphological analysis
Bi(2)O6 polyhedra
2.17(3)
Bi(2) O(2)
2.21(2)
Bi(2) O(2)
2.39(2)
83.5(8), 97.8(8)
82.7(8), 96.9(9)
156.3(8)
87.5(8)
80.9(8), 170.1(8),
102.1(9), 168.3(8)
77.5(8), 95.5(8)
89.3(9)
80.4(8), 99.4(9)
(Fig. 4, left). Four types of oxygen atoms could be distinguished in
the Bi2 WO6 structure. First one is O(3), which is present within the
layers of Bi(1) bridging four metal atoms. Second is O(2) situated
within the layers of Bi(2) which also bridged four metal cations like
O(3). Third kind of oxygen atoms – O(4) and O(5) – are lying within
the planes of WO4 2− layers and bridges 2 tungsten atoms. Finally
O(1) and O(6) bridges both the tungsten and bismuth layers where
O(1) is bonded to two Bi(2) atoms and O(6) is bonded to two Bi(1)
atoms, respectively [48]. The WO6 octahedra in Bi2 WO6 structure
are rather distorted and tilted along the [010] axis (Fig. 4). The distribution of W O bond lengths and O W O angles inside WO6
octahedra (see Table 2) shows that the tungsten atoms are shifted
away from the centres of octahedra in the direction of the polar axis.
The Bi2 O2 sheets are built up by edge sharing BiO4 square pyramids
where the oxygen atoms form basal planes and the Bi atoms occupy
the apex above and below the oxygen basal planes (Fig. 4, left). The
arrangement of Bi atoms above and below the oxygen squares is
caused by its stereo chemical activity due to the presence of 6s2
lone pair of electrons. Both crystallographically independent Bi1
and Bi2 atoms are bounded each with six oxygen atoms located at
the distances 2.17–2.56 Å (Table 2), but the disposition of oxygen
species in coordination polyhedra is strongly asymmetric (Fig. 4,
right). It is considered [48] that asymmetry of oxygen coordination
in BiO6 units may indicate the probable orientation of lone pairs,
which are directed into the WO6 layers (Fig. 4, right). The lone
pairs in the two Bi2 O2 2+ layers are directed into the sandwiched
tungsten layer in an asymmetric manner, which may be a reason for the tilting of WO6 octahedra observed in Bi2 WO6 structure
[48].
87.3(6)
65.5(7), 77.4(8),
77.7(8), 70.5(8)
73.3(9), 68.3(8),
76.7(9), 71.3(9)
Fig. 5 shows the FESEM image of 500 ◦ C calcined Bi2 WO6 and
the corresponding particle size histogram. The typical morphology
of the particles (Fig. 5a) is found to be spherical in shape and uniform size distribution of particles which is due to the application of
highly efficient sonochemical method used for the synthesis. The
reaction time plays a crucial role in the morphology of the material.
The uniform morphology may be also due to the longer reaction time of 2 h. The analysis of particle size distribution (Fig. 5b)
enumerates that the particle are found to be in the nano metre
range. The maximum number of particles are found to be around
724
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
Fig. 4. Bi2 WO6 structure as alternating WO6 and Bi2 O2 2+ layers (left) and coordination spheres of the tungsten and two crystallographically independent bismuth atoms in
Bi2 WO6 (right).
50–60 nm. The nanoparticle synthesis using sonochemical method
is based upon the principle of acoustic cavitation, i.e., formation,
growth and collapse of the bubble. During sonication in the liquid
medium, the bubbles are begins to grow which filled with both solvent and solute vapour. When these bubble reaches its maximum
volume, implosive compression in the cavities takes place leading
to produce local heating, high pressures and short life times [43].
Upon the collapse of the bubble, a very high temperature of the
order of >5000 K and pressure of >20 MPa could be achieved which
results in formation of particles with various structures containing
uniform distribution of particles [44]. Due to this high temperature and pressure, the water gets vaporized and further pyrolyzed
into H* and OH* radicals. The formation mechanism of Bi2 WO6
under ultrasonication was already reported by Zhang et al. [52].
The reaction mechanism for the formation of Bi2 WO6 is given by,
Bi(NO3 )3 ↔ Bi3+ + 3NO3 −
(1)
NO3 − + H2 O + Bi3+ → BiONO3 + 2H+
(2)
Na2 WO4 ↔ 2Na+ + WO4 2−
(3)
2BiONO3 + WO4 2− → Bi2 WO6 + 2NO3 −
(4)
Fig. 5c shows the corresponding EDX spectrum of Bi2 WO6 . It
elucidates the presence of constituent elements such as Bi, W and
O and the corresponding weight percentages are 57%, 21% and 20%,
respectively. The observed discrepancy in the atomic percentages
of Bi2 WO6 may be due to the presence of secondary phase of Bi2 O3 .
3.4. Electrical conductivity analysis
The conductivity measurements of Bi2 WO6 were analyzed
using the frequency dependence a.c.impedance spectroscopy. Fig. 6
shows the Cole–Cole plot of Bi2 WO6 measured at room temperature. As it is seen from the figure that there is a single depressed
semi-circle obtained at high frequency region and a tail is observed
in the low frequency region. The single semi-circle observed is contributed due to the parallel combination of bulk resistance (Rb )
and bulk capacitance (Cb ) which elucidates that the conductivity
mechanism in this material mainly arises due to the bulk of the
material (grain). The bulk resistance of the material is measured by
intercepting the semi-circle with the real axis and finding out the
diameter of the semi-circle. The diameter of the semi-circle gives
the value of bulk resistance and the value is found to be 4.6 × 106 .
Utilizing the value of bulk resistance, the d.c. conductivity value is
calculated using the relation
=
l
S cm−1
Rb A
(5)
where Rb is the bulk resistance (in Ohm), l is the thickness of
the pellet (in cm), A is the area of the pellet (in cm2 ). The
value of conductivity is calculated to be 3.96 × 10−8 S cm−1 .The
bulk capacitance of the material is calculated using the relation
2 max Rb Cb = 1. The value of capacitance found using this relation is 57 pF; this high value of capacitance reveals the conduction
mechanism to be material’s bulk contribution. Fig. 6 inset shows
the conductance spectrum of Bi2 WO6 measured at room temperature. As it is seen from the figure, there is a frequency independent
plateau at lower frequencies and a dispersive region at higher
frequencies. The frequency independent conductivity at the low
frequency region is equal to the bulk d.c. conductivity and it is due
to the activated hopping of charge carriers. The frequency independent region was switched over to the frequency dependent region
called the onset of conductivity relaxation at higher frequencies.
The spectra found obeys Jonscher’s power law given by the equation
[53]
(ω) = d.c + Aωn
(6)
where dc and A are thermally activated quantities; n is the
frequency exponent. By non-linearly fitting the conductance spectrum, the values of A and n are found to be 1.0155 × 10−12 and
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
725
Fig. 5. (a) FESEM images of Bi2 WO6 (b) Corresponding Particle size histogram (c) EDX spectrum of Bi2 WO6 .
1.539. The d.c conductivity ( dc ) of the material is found out to
be 3.87 × 10−8 S cm−1 . The obtained value of d.c. conductivity is in
consistence with the impedance spectral value.
10
3.5. Electrochemical studies
6
6
Z" x 10 , (Ohm)
8
4
2
0
0
2
4
6
8
10
6
Z' x 10 , (Ohm)
Fig. 6. Cole–Cole plot (inset: conductance spectrum) of Bi2 WO6 at room temperature.
3.5.1. Cyclic voltammetry
Cyclic voltammetry is the most important technique in electrochemistry which provides us the qualitative information regarding
the electrochemical processes that takes place in the material
i.e. whether Faradic or non-Faradic. Before entering into the
detail investigation of electrochemical performance of the material,
one must take some efforts in determining electrode/electrolyte
compatibility. The capacitance of the material arises due to the
adsorption/desorption of electrolyte ions into/from the electrode
material. The size and diffusion speed of the solvated ions in the
electrolyte plays a major role in determining the performance of
the material.
Fig. 7 shows the CV curves of Bi2 WO6 in four different electrolytes such as 1 M LiOH, 1 M NaOH, 1 M Na2 SO4 , 1 M KOH and 6 M
KOH at various scan rates. There is a pair of redox peaks observed for
Bi2 WO6 in all the electrolytes which elucidates the pseudo capacitive behaviour and these redox peaks are due to the oxidation
and reduction reactions takes place in the material. The reduction and the oxidation peak potential appeared in KOH electrolyte
726
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
Fig. 7. CV curves at various scan rates of (a) 1 M LiOH (b) 1 M NaOH (c) 1 M Na2 SO4 (d) 1 M KOH (e) 6 M KOH and (f) capacitance vs scan rate for different electrolytes.
are similar to the reported Bi2 O3 in hydroxide electrolyte [54]. The
reduction peaks around −0.87 V is due to the reduction of Bi (III) to
Bi metal and the oxidation peak around −0.67 V is due to the oxidation of Bi metal to Bi (III), respectively. The detailed mechanism
behind the oxidation and reduction process is described by Vivier
et al. [55,56] for Bi2 O3 . During the reduction process, the following
reaction takes place,
BiO2 − → BiO2 − (ads)
(7)
BiO2 − (ads) + e− → BiO2 2− (ads)
(8)
2H2 O + 3BiO2 2−
←→
2BiO2 − + 4OH− + Bi0
Disproportionation
Bi(0) → Bi(metal)
(9)
(10)
favours enhanced ionic mobility and interaction with the electrode
material, thereby resulting in enhanced electrochemical performance. The added advantage of K+ ions is that it acquires small
charge density i.e. weak solvation interactions with water molecule
that favours easier polarization during the de-solvation processes.
This causes an easy passage of K+ ions into the electrode during
the redox reactions. The above results substantiate that the cations
are having major role in the electrochemical reaction processes. In
order to understand the role of anions, CV analysis was carried out
using Na2 SO4 electrolyte and compared with NaOH electrolyte. It is
also seen that, the redox peak is dominant in NaOH compared with
Na2 SO4 electrolyte. This depressed phenomenon is mainly due to
the increased anionic size of sulphate ions (1.49 Å) compared with
the hydroxyl ions (1.10 Å).
The specific capacitances of the material in different electrolytes
are calculated using the relation,
During oxidation process, the following reaction takes place
Bimetal → Bi+ + e−
2Bi+
←→
(11)
Bi3+ + 2Bimetal
Disproportionation
(12)
3OH− + Bi3+ → Bi(OH)3
(13)
Bi(OH)3 → BiOOH + H2 O
(14)
It is noticeable from Fig. 7a–e, the redox behaviour and CV
integrated current area of Bi2 WO6 are high in KOH electrolyte compared with the LiOH and NaOH electrolyte that leads to a higher
capacitance. The current response of Bi2 WO6 in various electrolytes
decreases in the order of 6 M KOH > 1 M KOH > 1 M NaOH > 1 M
Na2 SO4 . This enhanced current response of KOH compared to other
electrolytes is due to the difference in the hydrated radius of K+
ions (3.31 Å), Na+ ions (3.58 Å) and Li+ ions (3.82 Å), respectively. It
is noted that the reported conductivity of K+ (73 cm2 / mol) ions is
greater than Na+ ions (50 cm2 / mol) and Li+ ions (38 cm2 / mol)
at 25 ◦ C [9,57] and hence the mobility of ions would be higher for
K+ compared with Na+ and Li+ . The lower hydrated radius of K+ ions
Specific capacitance =
I dv
2 × V × m × ϑ
(15)
where I dV represents the area under CV curve, V is the potential
window, m is the mass of the active material and ϑ is the scan
rate. According to above relation, the specific capacitance would
decrease on increasing scan rate.
The variations of specific capacitance value with scan rate for
different electrolytes are shown in Fig. 7f. The capacitance of the
material is found to be decreased on increasing scan rates and this
might be due to the number of active sites that are actively participated in the redox reactions may get decreased. The number
of active sites participating in the redox reactions is given by the
relation [58]
N=
C × M × V
F
(16)
where C is the specific capacitance (F/g), M is the molecular weight
(g/mol), V is the potential window (V), F is the Faraday’s constant
(96,500 C mol−1 ).
The calculated number of active sites involved in the redox reaction at different scan rates are 2.073, 1.900, 1.367, 0.843, 0.512,
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
ip = (2.687 × 105 )n3/2 ACD1/2 ϑ1/2
800
(a)
700
600
Capacitance, F/g
and 0.3182 corresponding to 5 mV/s, 10 mV/s, 20 mV/s, 30 mV/s,
40 mV/s, 50 mV/s and 100 mV/s, respectively in 1 M KOH. The
number of redox sites participating at lower scan rates is higher
compared with the higher scan rates. At slow scan rates, the ions
would have enough time to arrive the electrode surface leading to
the full utilization of the material. At higher scan rates, the ions
would not have enough time to utilize the material and hence the
surface adsorption process only takes place [10].
Compared with 1 M NaOH and 1 M LiOH electrolyte, the 1 M
KOH electrolyte provides the maximum capacitance due to the easy
passage of K+ ions into the inner surface of the electrode material.
The diffusion coefficient of Bi2 WO6 were calculated using Randle’s
Sevick equation for reversible (Eq. (17)) and irreversible (Eq. (18))
systems for 1 M NaOH and 1 M KOH is given by
500
400
300
200
100
(17)
0
0
RT
ϑ1/2
where, n and n␣ represents the number of electrons transferred
during redox reaction, F is faradays constant, ␣ is transfer coefficient, R gas constant, T temperature, A is the area of the electrode (in
cm2 ), D is the diffusion co-efficient (cm2 /s), C is the concentration
(in mol/cm3 ), ϑ is the scan rate (mV/s) and ip is the peak current. The
value of diffusion co-efficient depends mainly on the peak current
since the other parameters in the equation such as the number of
electrons transferred during the redox reaction, concentration and
scan rate remains same for all other electrolytes. According to the
above equation, the peak current obtained at various scan rates will
be proportional to the square root of the scan rate. Since the peak
current is higher for 1 M KOH electrolyte, the value of diffusion
co-efficient is found to be higher for 1 M KOH (3.6 × 10−17 cm2 /s)
compared with 1 M NaOH (1.2 × 10−17 cm2 /s) at 2 mV/s.
In order to further enhance the specific capacitance of Bi2 WO6
in 1 M KOH electrolyte, the concentration of the ions in the electrolyte was raised to 6 M. Bi2 WO6 supercapacitor material in 6 M
KOH electrolyte acquires a maximum capacitance among all the
electrolytes as evident from Fig. 7a–e. Compared with 1 M KOH, 6 M
KOH electrolytes possess a high current value that leads to higher
capacitance. This happens since the number of ions participating in
the redox reactions gets increased. The stability of the material in
6 M KOH electrolyte is studied by cycling the material to 200 cycles.
It should also be noted that in Fig. 8a there is a large capacitance fading observed when the material is cycled for 200 cycles at a scan rate
of 5 mV/s. There is a serious capacitance reduction and fading on
cycling occurs. This capacity fading is accompanied with the dissolution of the active material into the electrolyte and corrosion of the
current collector. Fig. 8a (inset) shows the XRD pattern of Bi2 WO6
taken after 200 cycles in 6 M KOH electrolyte. The observed peaks
in the diffraction pattern correspond to the (Bi2 O2 ) (W2 O7 ) (JCPDS
No. 89-8114). There is a phase change occurred from Bi2 WO6 to
(Bi2 O2 ) (W2 O7 ) phase which is evident from the XRD pattern.
The electrochemical stability of Bi2 WO6 in 1 M KOH electrolyte
was investigated by CV measurements at a scan rate of 10 mV/s for
100 cycles. As observed from Fig. 8b, the initial capacitance of the
material is found to be 269 F/g and the capacitance gets increased
to 333 F/g upon cycling to 24th cycle. The initial increased capacitance is due to the activation effect [59] and the capacitance gets
maintained on further cycling. After 100 cycles, the capacitance is
found to be 295 F/g. The capacity retention is found to be 89% after
100 cycles which is measured by eliminating the capacitance due to
the activation effect. The electrochemical stability of the electrode
material in 1 M KOH is found to be superior when compared with
6 M KOH electrolyte and hence the further charge discharge study
is employed using 1 M KOH electrolyte.
50
100
150
200
Cycle number
600
(18)
(b)
450
Capacitance, F/g
ip = 0.4958nFACD
1/2
1/2 ˛n˛ F
727
300
150
0
0
20
40
60
80
100
Cycle number
Fig. 8. (a) Capacity fading in 6 M KOH on cycling (insert: XRD pattern of Bi2 WO6
taken after 200 cycles in 6 M KOH electrolyte). (b) CV cycling stability of Bi2 WO6 in
1 M KOH at a scan rates 10 mV/s.
Fig. 9 shows the Trasatti plot of Bi2 WO6 in 1 M KOH at 0.4, 0.6,
0.8, 1, 1.2, 1.4, 1.6 and 1.8 mV/s. The amount of charge stored at the
inner and outer surface of electrode material could be estimated
using this plot. It is well known that the charge storage mechanism is sweep rate dependent. According to Trasatti [60], the total
amount of charge stored in the material is mainly depend upon
both the contribution of inner and outer surfaces and is given by
q(Total) ∗ = q(Inner) ∗ +q(Outer) ∗
(19)
where, q(Inner) * represents the inner surface and q(Outer) * is the outer
surface.
At higher scan rates, q* (voltammetric charge) is due to the easily accessible outer surface whereas at lower scan rates, q* is due
to the contribution of both inner and outer surfaces. The q(Outer) *
dependence on the scan rate is given by,
q∗ (ϑ) = q∗∞ + constant(ϑ−1/2 )
(20)
The value of q* is obtained by extrapolating the straight line at
ϑ−1/2 = 0 to y-axis. The amount of charge that is stored at the outer
surface of the electrode material can be found by linearly fitting the
plot of q* vs ϑ−1/2 and the value is found to be 223 C/g.
728
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
3.0
700
(a)
Bi2W O6
2.8
2.5
500
2.0
1.8
400
-1
q* (C.g )
-1
-3
Linear Fit
600
2.3
1/q* x 10 (C .g)
(b)
Bi 2W O 6
Linear Fit
1.5
1.3
-3
-1
Intercept (1/q*)Total = 0.9197 x 10 C .g
1.0
200
0.8
q*Total = 1087 C.g
0.5
Intercept q*outer = 223 C.g
300
Since, q*Total = q*outer + q*innier
-1
q*Inner = 864 C.g
100
0.3
0.0
-1
-1
0
0.0
0.2
0.4
1/2
0.6
1/2
0.8
(mV .s
-1/2
1.0
1.2
1.4
0.0
0. 2
0. 4
0.6
)
0.8
-1/2
(mV
1.0
-1/2
1. 2
1.4
1.6
1.8
1/2
.s )
Fig. 9. Trasatti plot using 1 M KOH at various scan rates.
The total amount of charge that is stored in electrode material can be found using the plot of 1/q* vs ϑ1/2 . The 1/q* decreases
linearly with ϑ1/2 given by the relation
1
1
(ϑ) =
+ constant(ϑ1/2 )
q∗
qo ∗
(21)
By linearly fitting the 1/q* vs ϑ1/2 curve, the value is found to be
1087 C/g. Using the relation (19), the amount of charge that is stored
in the inner surface is calculated to be 864 C/g. From the above
results, it could be concluded that the amount of charge that is
stored at the inner surface of the material has higher contribution
to the electrochemical performance than the outer surface [61].
3.5.2. Electrochemical impedance spectral (EIS) analysis
Nyquist plot is a plot relating the real part of impedance (Z ) vs
imaginary part of impedance (−Z ). The Nyquist plot of Bi2 WO6
nanoparticles in 1 M NaOH, 1 M LiOH and 1 M KOH electrolytes
are shown in Fig. 10. A similar type of behaviour with semi-circle
in the mid-high frequency and a linear region namely Warburg
impedance at the low frequency region is observed at all the
electrolytes. The Warburg impedance is due to the frequency
dependent ionic diffusion of ions from electrolyte into the electrode material. The linear region exhibits an angle of about 60◦
with respect to the real axis indicating that the electrode process
are not perfectly capacitive but it is diffusion control [9] since the
ideal polarizable capacitor is characterized by a straight line in the
7
1M KOH
1M NaOH
1M LiOH
6
4
3
-Z" x 10 (Ohm)
5
3
2
1
0
0
1
2
3
3
4
5
6
7
Z' x 10 (Ohm)
Fig. 10. EIS spectrum of Bi2 WO6 in 1 M KOH, 1 M NaOH and 1 M LiOH electrolyte.
low frequency which is parallel to the imaginary axis [62]. The
semi-circle in the mid-high frequency corresponds to the parallel
combination of charge transfer resistance (Rct ) and double layer
capacitance (Cdl ). The EIS spectra is fitted using Z-fit equivalent
circuit having the components of solution resistance (Rs ), charge
transfer resistance (Rct ), double layer capacitance (Cdl ) and Warburg impedance (Zw ). The charge transfer resistance (Rct ) is found
by intercepting the semi-circular arc at the real-axis. As it is seen
from Fig. 10, the Rct value is found to be smaller for KOH (67 )
compared with NaOH (94 ) which implies that the charge transfer at the electrode/electrolyte interface is facile in the case of KOH
compared to NaOH electrolyte. The value of double layer capacitance (Cdl ) is found to be 45 × 10−6 F for 1 M KOH, 34 × 10−6 F for 1 M
NaOH and 22 × 10−6 F for 1 M LiOH, respectively. In addition to this,
the electrolyte resistance (Rs ) of NaOH (1.604 ), LiOH (1.833 )
are found to be larger compared to KOH electrolyte (1.522 ). The
value of Rs is the contribution of ohmic resistance of the electrolyte,
internal resistance of the electrode material and contact resistance
at the electrode/current collector interface. This elucidates the conception that the equivalent series resistance (ESR) is lower for KOH
electrolyte. As it is known, the power density of the material is
related to the ESR value given by the relation
Power density =
U2
4 × ESR
(22)
The Bi2 WO6 materials in KOH electrolyte possess lower ESR value
and high rate capability, i.e. high power density compared with
NaOH solution.
3.5.3. Charge–discharge studies
Among the electrolyte systems selected for studies, Bi2 WO6
material shows an enhanced capacitive and stability performance
in 1 M KOH electrolyte and hence to acquire more information regarding the electrochemical properties of the material,
charge–discharge studies are employed using 1 M KOH in the
potential window between −0.9 and 0.1 V. The charge discharge curves of Bi2 WO6 at various current densities such as
0.5 mA/cm2 , 1 mA/cm2 , 2 mA/cm2 , 3 mA/cm2 are shown in Fig. 11.
The non-linear characteristic of the discharge curve infers that the
electrochemical reaction happens due to the redox mechanism and
this result is in concordant with the CV results. A non-linear horizontal charge discharge behaviour observed in our study depicts
the curve that is observed in the case of bismuth molybdate by Liu
et al. [63].
As seen from the figure, a higher discharge time is observed
for the material at low current density and the discharge time
decreases on increasing current density. The higher is the discharge
V.D. Nithya et al. / Electrochimica Acta 109 (2013) 720–731
729
500
Specific capacitance, F/g
Potential, V Vs Hg/HgO
-0.2
120
2
-0.4
-0.6
100
400
80
300
60
200
40
100
Coulombic efficiency,
0.5 mA/cm
2
1 mA/cm
2
2 mA/cm
2
3 mA/cm
0.0
20
-0.8
0
0
200
400
600
800
1000
1200
1400
1600
0
0
1800
100
Fig. 11. Galvanostatic charge–discharge curves at various current densities in 1 M
KOH.
time, larger is the specific capacitance. Hence, the specific capacitance is larger at lower current density and decreases with
increasing current density. The discharge behaviour of Bi2 WO6
observed in our case is found to be asymmetrical and non-linear, the
specific capacitance of the material is calculated using the relation
[64–66]
2E
M · V22 − V12
(23)
where E is the energy density, M is the total mass of the active
material, V2 is the maximum voltage (0.1 V) and V1 is the minimum
voltage (−0.9 V).
The specific capacitance value at the current densities of
0.5 mA/cm2 , 1 mA/cm2 , 2 mA/cm2 and 3 mA/cm2 are 608 F/g,
427 F/g, 339 F/g and 304 F/g, respectively. The specific capacitance
if found to decrease with increasing current density. This is due to
increase in ionic resistivity and decrease in charge diffusion deeper
into the inner active sites [12]. The fading at the higher current
densities are mainly due to larger voltage (IR) drop and it is the
common phenomena occurring in the transition metal oxides [67].
The energy density E (in Wh/kg) and the power density P (in W/kg)
of the Bi2 WO6 nanoparticles at various current densities are calculated using the equations [64–66]
E=
P=
I
V (t) dt
M
E
t
(24)
(25)
300
400
500
Cycle number
Time, s
Csp =
200
Here, V(t)dt is the integral area of the discharge curve, I is the
current density, M is the mass of the active material and t is the
discharge time. The calculated energy density values at current
densities of 0.5 mA/cm2 , 1 mA/cm2 , 2 mA/cm2 and 3 mA/cm2 are
67 Wh/kg, 47 Wh/kg, 38 Wh/kg and 34 Wh/kg, respectively. Using
the values of energy density, the power density is found to be
293 W/kg, 591 W/kg, 1120 W/kg and 1790 W/kg for 0.5 mA/cm2 ,
1 mA/cm2 , 2 mA/cm2 and 3 mA/cm2 , respectively. As the current
density is increased, energy density gets decreased, i.e. the amount
of charges that gets stored per unit volume/mass gets declines on
current density. The power density gets increased, i.e. the rate of
charge/discharging per unit mass/volume increases on increasing
current density.
The electrochemical stability of the Bi2 WO6 nanoparticles is
analyzed using galvanostatic charge discharge studies. Fig. 12
shows the cycling stability and coulombic efficiency of Bi2 WO6 at
Fig. 12. Charge–discharge cycling stability curve at a current density of 3 mA/cm2
and the corresponding columbic efficiency.
a current density of 3 mA/cm2 for 500 cycles. The initial discharge
capacitance of the material is found to be 304 F/g. There is a gradual decrease in the capacitance value on further cycling. The value
of capacitance is 239 F/g after 100 cycles and it further decreases
to 207 F/g during 500 cycles. After 500 cycles, it maintains only
68% of the initial capacitance. This capacitance fading on cycling
might be due to the presence of minor traces of secondary impurity
Bi2 O3 . The coulombic efficiency of the material at a current density
of 3 mA/cm2 is found to be 100% and it maintains throughout the
cycles.
4. Conclusion
Bi2 WO6 nanoparticles were successfully synthesized by sonochemical method. The electrochemical behaviour of the material
was investigated in various aqueous electrolytes such as 1 M NaOH,
1 M LiOH, 1 M Na2 SO4 and 1 M, 6 M KOH. The structural investigation suggested the material was highly crystalline and spherical
in morphology. The material possesses room temperature conductivity and the conductivity value was around 3.9 × 10−8 S cm−1 . The
cyclic voltammetric results suggested that among all the electrolyte
systems studied, KOH electrolyte possess higher current response
leading to higher capacitance. The charge–discharge study in 1 M
KOH electrolyte at a current density of 3 mA/cm2 was found to be
304 F/g. The coulombic efficiency was found to be 100% and it was
maintained throughout the cycles. The reason for the enhanced
electrochemical performance of Bi2 WO6 in KOH electrolyte was
due to the smaller hydration sphere radius of K+ ions compared
to Na+ and Li+ ions. Also, the ionic conductivity of K+ ions was
higher compared with Na+ and Li+ ions. This lower hydration radius
and higher ionic conductivity led to enhanced performance of the
material in 1 M KOH electrolyte.
Acknowledgements
One of the authors (V.D. Nithya) deeply thanks Department of
Science and Technology (DST), New Delhi, India for providing fellowship under DST-INSPIRE program. L. Vasylechko acknowledges
a partial support of the Ukrainian Ministry of Education and Sciences (Project “Neos”).
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